This invention relates generally to filtering electronic signals and more particularly to a method and apparatus for filtering a signal and producing a response having a boost and asymmetric zeros and hard disk drive incorporating same.
Filters used in high-frequency applications, such as disk drive, video, and data transmission applications are generally implemented as continuous-time active filters. These continuous-time active filters are often implemented using transconductance-capacitor (xe2x80x9cgmxe2x88x92Cxe2x80x9d) filters. Continuous-time active filters of a desired order may be constructed by serially coupling or connecting gmxe2x88x92C filter stages until a filter having the desired order and response is provided. For example, continuous-time active filters configured as second order filters may be provided using gmxe2x88x92C filter stages. A second order filter is one whose transfer function contains complete quadratic equations in both the numerator and the denominator and can be implemented, for example, as a low pass filter, a high pass filter, or a notch filter.
In many applications, such as in data transmission and disk drive applications, it is advantageous to provide a continuous-time active filter with a constant group delay characteristic over a desired range of frequencies to prevent distortion of a signal waveform. The group delay may be defined as the negative of the derivative of the phase with respect to frequency. Furthermore, it is often advantageous and desirable to provide an adjustable or selectable group delay characteristic. It is also often advantageous and desirable to provide amplification of selected frequency components or frequency spectrums. The amplification of selected frequency components or frequency spectrums may be referred to as xe2x80x9cboost.xe2x80x9d
For illustration purposes, the standard second order low pass filter transfer function is provided below:       H    ⁡          (      s      )        =            ω      o      2                      s        2            +              s        ⁢                  xe2x80x83                ⁢                              ω            o                    Q                    +              ω        o        2            
In order to provide an adjustable group delay that will be constant throughout the frequency spectrum, and boost, the standard second order low pass transfer function may be converted into the following equation:       H    ⁡          (      s      )        =                              -          k                ⁢                  xe2x80x83                ⁢                  s          2                    +              b        ⁢                  xe2x80x83                ⁢        s            +              ω        o        2                            s        2            +              s        ⁢                  xe2x80x83                ⁢                              ω            o                    Q                    +              ω        o        2            
where the term xe2x80x9cxe2x88x92ks2xe2x80x9d is a boost term, and the term xe2x80x9cbsxe2x80x9d is an asymmetric zero term which is used to provide the adjustable group delay. The boost term, xe2x88x92ks2, increases high frequency gain by adding two real symmetric zeros to the transfer function. Since one of the zeros is positive and the other is negative, and the two are of equal magnitude, the phase is not changed. The amplitude of the boost may be programmable and may be adjusted according to the value of xe2x80x9ck.xe2x80x9d The asymmetric zero term, bs, causes the real zeros to no longer be symmetric which results in a change in the phase and hence the group delay of the transfer function. In order to provide asymmetry, b may be provided as either a positive value or a negative value, but not at a zero value. Thus, the group delay may be adjusted by changing the xe2x80x9cbxe2x80x9d term of the asymmetric zero term.
In disk drive applications, the boost may be used for such applications as pulse slimming and/or read channel equalization. The asymmetric zeros or adjustable group delay may be used in such applications to adjust the group delay characteristics of the disk drive read channel or data channel to optimize performance.
Prior attempts at providing boost and adjustable group delay have proven unsatisfactory at best. For example, one prior attempt at providing boost and adjustable group delay involved the use of an amplifier for amplifying an input signal to a gmxe2x88x92C filter stage and driving the bottom plate or electrode of the output capacitor of the gmxe2x88x92C filter stage. This presented serious technical problems due to the existence of a parasitic capacitor whose bottom plate could not be driven by the amplifier. In some cases, the value of the parasitic capacitor was up to thirty percent of the value of the output capacitor of the gmxe2x88x92C filter stage. As a result, the effective boost term, ks2, was greatly reduced. Because the boost term was significantly reduced, an amplifier having, a very high gain had to be provided to overcome this limitation. The high gain amplifier significantly increased overall power consumption. Also, the value of the parasitic capacitance was difficult, if not impossible, to predict because of semiconductor fabrication variations. This required the use of an amplifier with adjustable gain, which further increased circuitry and costs. Furthermore, the amplifier generally had a limited bandwidth, which reduced its effectiveness for applications using high frequency signals.
Another prior attempt at providing boost involved amplifying a current provided through the capacitor of a gmxe2x88x92C filter stage of a continuous-time filter. This technique has also proven unsatisfactory. The current provided through the capacitor is often large and consumes a significant amount of power when amplified. The increased power consumption results in increased circuitry area to handle the increased power consumption. The increased circuitry area increases undesirable circuitry parasitics.
From the foregoing it may be appreciated that a need has arisen for a method and apparatus for providing boost and asymmetry in a continuous-time filter that eliminate or reduce the problems described above. In accordance with the present invention, a method and apparatus for filtering a signal and producing a response having boost and asymmetrical zeros and hard disk drive incorporating same are provided that address the disadvantages and problems outlined above.
According to one embodiment of the invention, an apparatus for filtering an input signal includes a first second order filter section having an output and an intermediate output, a second second order filter section having an input connected to the output of the first second order filter, and a gain stage coupling the output of the first second order filter section and the intermediate output of the first second order filter section to the output of the second second order filter section. The gain stage is coupled such that a transfer function between the output of the second second order filter section and the input of the second second order filter section is a biquadratic transfer function.
According to another embodiment of the invention, a hard disk drive system includes a rotating magnetic disk having information magnetically stored thereon, a read head supported adjacent said disk and operative to read information from said disk and to output an analog signal embodying the information, and a read channel circuit that has an input coupled to the analog signal, the read channel including a filter for filtering a signal indicative of the analog signal. The filter includes a first second order filter section having an output and an intermediate output, a second second order filter section having an input connected to the output of the first second order filter, and a gain stage coupling the output of the first second order filter section and the intermediate output of the first second order filter section to the output of the second second order filter section. The filter is coupled such that a transfer function between the output of the second second order filter section and the input of the second second order filter section is a biquadratic transfer function.
Embodiments of the invention provide numerous technical advantages. For example, in one embodiment of the invention, a filter is provided that has a boost and asymmetric zeros that may be specified without otherwise affecting the response of the filter. In addition, such boost and asymmetric zeros are provided without suffering the above-described problems associated with driving the bottom plate of a capacitor.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.